1. Field of the Invention
The field of this invention is vertebrate netrin proteins and genes which are involved in neural axon outgrowth.
2. Background
In the developing nervous system, axons project considerable distances along stereotyped pathways to reach their targets. Axon growth and guidance depends partly on the recognition of cell-surface and extracellular matrix cues along these pathways. The identification of such nerve cell growth and guidance cues is the holy grail of neurobiology. These are the compounds that tell neurons when to grow, where to grow, and when to stop growing. The medical applications of such compounds are enormous and include modulating neuronal growth regenerative capacity, treating neurodegenerative disease, and mapping (e.g. diagnosing) genetic neurological defects.
Over decades of concentrated research, various hypotheses involving chemo-attractants and repellents, labeled pathways, cell adhesion molecules, etc. have been invoked to explain guidance. Molecules such as N-CAM and N-cadherin have been reported to provide favorable substrates for axon growth and certain sensory axons may be responsive to NGF and NGF-like factors. Recent reports suggest the existence of diffusible chemotropic molecule(s) which influence the pattern and orientation of commissural axon growth.
Relevant Literature
Placzek et al. (1990) Development 110, 19-30; Placzek et al. (1990) Cold Spring Harbor Symposia on Quantitative Biology 55, 279-302.; and Tessier-Lavigne et al. (1988) Nature 336: 775-778 report evidence for diffusible chemotropic molecules which influence the pattern and orientation of commissural axon growth. Gundersen and Barret (1980) JCB 87, 546-554, Lohof et al. (1992) J. Neurosci. 12 (4), 1253-1261 and Zheng et al. (1993) Soc. Neurosci. Abstr 19, 608.9 report neural chemotaxis in response to NGF, cAMP and acetylcholine, respectively. Ishii et al. (1992) Neuron 9, 873-881 disclose a gene, unc-6, derived from C. elegans, which has sequence similarity to the disclosed netrins. Data disclosed in this application was published in Serafini et al (1994) Cell 78, 409-424 and Kennedy et al (1994) Cell 78, 425-435 at page 5, column 1. The work was also reported in The New York Times, Section B7, Tuesday, Aug. 16, 1994 and more recently (May 19, 1995) described in Science 268, 971-973 (see also references cited therein).
The invention provides methods and compositions relating to netrins and netrin genes. Netrins are a novel class of proteins which are naturally involved in neural axon guidance. The subject compositions include nucleic acids which encode netrin proteins and hybridization probes and primers capable of hybridizing with netrin genes. Netrins find particular use in modulating neural axon outgrowth. The disclosed compositions also find use variously in screening chemical libraries for regulators of axon outgrowth and orientation, in genetic mapping, as probes for related genes, as diagnostic reagents for genetic neurological disease and in the production of specific cellular and animal systems for the development of neurological disease therapy.
The invention provides methods and compositions relating to netrins and netrin genes; including methods and compositions for identifying, purifying, characterizing, and producing netrins and for identifying, characterizing, cloning, expressing, inhibiting the expression of and amplifying netrin genes.
Netrins are characterized by sequence similarity to the disclosed netrins 1 and 2. Using the amino acid sequence search program BLASTP (Altschul et al. (1990) Basic Local Alignment Search Tool, J Mol Biol 215, 403-410), complete (full length) netrin amino acid sequences provide a Probability P(N) score of less than 1.0 exe2x88x92200. In contrast, complete amino acid sequence comparison of a netrin with the evolutionarily related laminin proteins provides P(N) scores exceeding 1.0 exe2x88x92144. In addition, netrins generally show at least about 25% overall pair-wise sequence identity with all of the disclosed netrins 1 and 2 and at least about 50% pair-wise sequence identity within domain V. Furthermore, netrins are generally characterized by netrin-specific amino acid sequences invariant across the disclosed netrins 1 and 2 as seen in their amino acid alignments. The subject netrins may be incomplete translates of the disclosed netrin cDNA sequences or deletion mutants of the corresponding conceptual translates, which translates or deletion mutants have the netrin binding activity and specificity described herein.
Netrin peptides of the invention comprise unique portions of the disclosed netrin polypeptides and netrin receptors. A xe2x80x9cunique portionxe2x80x9d has an amino acid sequence unique to that disclosed in that it is not found in any previously known protein and has a length at least long enough to define a novel peptide. Unique portions are found to vary from about 5 to about 25 residues, preferably from 5 to 10 residues in length, depending on the particular amino acid sequence and are readily identified by comparing the subject portion sequences with known peptide/protein sequence data bases. Preferred unique portions include netrin residues that directly bind and activate (agonize) netrin receptors, especially residues that derive from the EGF-like domains of the disclosed sequences, especially those of the human varieties.
Particular preferred netrin peptides are listed here. These peptides are shown by functional assays disclosed herein to have biological activity including axon outgrowth and/or orienting activity. It is apparent to those of ordinary skill in the art that substitutions of chemically conservative residues can be made while preserving function.
The claimed netrins are isolated, partially pure or pure and are typically recombinantly produced. An xe2x80x9cisolatedxe2x80x9d protein for example, is unaccompanied by at least some of the material with which it is associated in its natural state and constitutes at least about 0.5%, preferably at least about 2%, and more preferably at least about 5% by weight of the total protein in a given sample; a partially pure protein constitutes at least about 10%, preferably at least about 30%, and more preferably at least about 60% by weight of the total protein in a given sample; and a pure protein constitutes at least about 70%, preferably at least about 90%, and more preferably at least about 95% by weight of the total protein in a given sample. A wide variety of molecular and biochemical methods are available for generating and expressing the subject compositions, see e.g. Molecular Cloning, A Laboratory Manual (Sambrook, et al. Cold Spring Harbor Laboratory), Current Protocols in Molecular Biology (Eds. Aufubel, et al., Greene Publ. Assoc., Wiley-Interscience, NY) or that are otherwise known in the art. The disclosed netrin peptides are also used as immunogens to generate specific polyclonal or monoclonal antibodies. See, Harlow and Lane (1988) Antibodies, A Laboratory Manual, Cold Spring Harbor Laboratory, for general methods.
The disclosed netrin compositions may be used to modulate axon outgrowth or guidance in situ or in vivo. For in vivo applications, the compositions are added to a retained physiological fluid such as blood or synovial fluid. For CNS administration, a variety of techniques are available for promoting transfer of the therapeutic across the blood brain barrier including disruption by surgery or injection drugs which transiently open adhesion contact between CNS vasculature endothelial cells, and compounds which facilitate translocation through such cells. Netrins may also be amenable to direct injection or infusion, topical, intratracheal/nasal administration e.g. through aerosol, intraocularly, or within/on implants e.g. fibers e.g. collagen, osmotic pumps, grafts comprising appropriately transformed cells, etc. A particular method of administration involves coating, imbedding or derivatizing fibers, such as collagen fibers, protein polymers, etc. with therapeutic proteins. Other useful approaches are described in Otto et al. (1989) J Neuroscience Research 22, 83-91 and Otto and Unsicker (1990) J Neuroscience 10, 1912-1921. Generally, the amount administered will be empirically determined, typically in the range of about 10 to 1000 xcexcg/kg of the recipient and the concentration will generally be in the range of about 50 to 500 xcexcg/ml in the dose administered. Other additives may be included, such as stabilizers, bactericides, etc. will be present in conventional amounts.
The invention provides netrin-specific binding agents including isolated binding targets such as membrane-bound netrin receptors and netrin-specific antibodies and binding agents identified in screens of natural and synthetic chemical libraries, and methods of identifying and making such agents, and their use in diagnosis, therapy and pharmaceutical development. Generally, netrin-specificity of the binding agent is shown by binding equilibrium constants. Such agents are capable of selectively binding a netrin, i.e. with an equilibrium constant at least about 107 Mxe2x88x921, preferably at least about 108 Mxe2x88x921, more preferably at least about 109 Mxe2x88x921. A wide variety of cell-based and cell-free assays may be used to demonstrate netrin-specific binding; preferred are rapid in vitro, cell-free assays such as mediating or inhibiting netrin-cell/protein binding, immunoassays, etc.
The invention also provides nucleic acids encoding the subject proteins, which nucleic acids may be part of netrin-expression vectors and may be incorporated into recombinant cells for expression and screening, transgenic animals for functional studies (e.g. the efficacy of candidate drugs for neural disease or injury), etc. and nucleic acid hybridization probes and replication/amplification primers having a netrin cDNA specific sequence. The hybridization probes contain a sequence common or complementary to the corresponding netrin gene sufficient to make the probe capable of specifically hybridizing to the corresponding netrin gene in the presence of laminin genes. Hybridization probes having in excess of 100 continuous bases of netrin gene sequence are generally capable of hybridizing to the corresponding netrin cDNA and remaining bound at a reduced final wash stringency of 0.2xc3x97SSC (0.9 M saline/0.09 M sodium citrate) and 0.1% SDS buffer at a temperature of 65xc2x0 C.
Netrin genes, the term including natural genomic and mRNA/cDNA sequences, are characterized by sequence similarity to the disclosed netrin 1 and 2 cDNAs. Using the nucleic acid sequence search program BLASTX (Altschul et al. (1990) Basic Local Alignment Search Tool, J Mol Biol 215, 403-410), complete coding region (full length) netrin cDNA sequences provide a Probability P(N) score of less than 1.0 exe2x88x92200. In contrast, complete coding region nucleic acid sequence comparison of a netrin cDNA with the evolutionarily related laminin cDNAs provides P(N) scores exceeding 1.0 exe2x88x92144. In addition, netrin cDNAs generally show at least about 25% overall coding region pair-wise sequence identity with the disclosed netrins 1 and 2 cDNAs and at least about 35% domain V coding region pair-wise sequence identity. Furthermore, netrin genes are generally characterized by netrin gene-specific nucleic acid sequences invariant across the disclosed netrin 1 and 2 cDNAs as seen in their nucleic acid alignments. Vertebrate netrin genes derive from vertebrates.
The subject nucleic acids are isolated, meaning they comprise a sequence joined to a nucleotide other than that which it is joined to on a natural chromosome and usually constitutes at least about 0.5%, preferably at least about 2%, and more preferably at least about 5% by weight of total nucleic acid present in a given fraction. A partially pure nucleic acid constitutes at least about 10%, preferably at least about 30%, and more preferably at least about 60% by weight of total nucleic acid present in a given fraction. A pure nucleic acid constitutes at least about 80%, preferably at least about 90%, and more preferably at least about 95% by weight of total nucleic acid present in a given fraction. The subject nucleic acids find a wide variety of applications including use as translatable transcripts, hybridization probes, PCR primers, therapeutic nucleic acids, etc.; use in detecting the presence of netrin genes and gene transcripts, in detecting or amplifying nucleic acids encoding other netrins, and in gene therapy applications, e.g. antisense oligonucleotides capable of inhibiting the intracellular expression of a targeted netrin transcript.
The invention provides efficient methods of identifying pharmacological agents or lead compounds for agents capable of mimicking or modulating netrin function (e.g. bioactive netrin deletion mutants and netrin peptides). A wide variety of screens may be used; for example, cell-based assays for may be used for monitoring netrin function and in vitro binding assays may be used to identify netrin-specific binding agents. Tessier-Lavigne et al. (1988, supra) describe an assay for netrin activity and Kennedy et al. (1994) Cell 78, 425-435 describe a particularly convenient COS cell-based netrin expression assay. Preferred methods are amenable to automated, cost-effective high throughput screening of natural and synthetic chemical libraries for lead compounds. Identified reagents find use in the pharmaceutical industries for animal and human trials; for example, the reagents may be derivatized and rescreened in in vitro and in vivo assays to optimize activity and minimize toxicity for pharmaceutical development.
Vertebrate netrin genes are cloned using the using the two general cloning strategies illustrated below for mouse and human netrins. First, using a strategy based on the initial amplification of a PCR product, oligonucleotide primers are designed using amino acid and nucleic acid sequences conserved among the previously identified vertebrate netrin sequences. Using these primers, a partial cDNA clone, corresponding to the novel netrin of interest is amplified from cDNA ergonomic DNA from the tissue and organism of interest by PCR. This partial clone is then used to generate a labeled probe which is used to screen a cDNA library or genomic library at high stringency to isolate a full length cDNA corresponding to the clone of interest. We describe below how such a strategy, based on PCR followed by library screening, has been used to successfully isolate mouse netrin-1,Drosophila netrin-a, and two human netrin cDNAs. The second general strategy utilizes reduced stringency library screening (Sambrook et al., 1989). We demonstrate below the applicability of this method in the isolation of mouse netrin-2. In this case we amplified and incorporated 32P into a probe which corresponded to domains VI and V in chicken netrin-2. Domains VI and V contain a number of regions of sequence which are well conserved among all vertebrate netrin family members isolated to this date. This probe was then used to screen an embryonic mouse brain cDNA library at reduced stringency. Our cloning of mouse netrin-2 using this method demonstrates that hybridization conditions are conveniently established which will detect netrin sequences between vertebrate species while avoiding significant background hybridization to non-netrin clones.
Our data identify netrin sequences common to the vertebrate netrins, mouse netrin-1, chicken netrin-1, and chicken netrin-2, which are not shared by the invertebrate netrin unc-6 as seen in netrin sequence alignments. The presence of these sequences, specific to vertebrates and conserved in all vertebrate netrins isolated, provides the necessary and sufficient sequence informative for generating primers and/or probes for any vertebrate netrin gene. In addition, amino acid sequence alignments similarly demonstrate that the vertebrate netrins define a structural class sharing common sequences not shared-with the invertebrate species illustrated by C. elegans unc-6 and the Drosophila Melanogaster netrin-a. Furthermore the alignment between the Drosophila and the C, elegans sequences indicates there is a greater diversity of netrin amino acid sequence represented within the invertebrate phylum than is present within the sequences derived from the vertebrate phylum.
The following examples are offered by way of illustration and not by way of limitation.